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By Alex Johnson

The Amtrak train that crashed in north Philadelphia this week was accelerating until it was shooting along the track at more than double the speed limit as it approached the curve where it derailed, safety investigators said Thursday.

Robert Sumwalt, a member of the National Transportation Safety Board, told reporters that investigators have no idea why Amtrak's Northeast Regional Train 188 was racing so fast — preliminary tests and inspections indicate that the train was on time and that there was nothing wrong with its headlights, its signals or the track.

At least eight people on the train were killed Tuesday night, and scores more of the 243 people on board were injured.

Sumwalt said investigators had been able to piece together a timeline of the moments leading up to the derailment based on the train's track image recorder, which is at the front of the locomotive.

One minute and five seconds before the recording ends, the train was traveling at 70 mph — and then it sped up, he said:

At 43 seconds before the recording ends, the train was up to 80 mph.

Twelve seconds later, it hit 90 mph.

And 15 seconds after that, it topped 100 mph, before engineer Brandon Bostian engaged the emergency brake.

"Mere seconds into the turn, we could see the train tilting approximately 10 degrees to the right and then the recording went blank," Sumwalt said.

The big mystery is whether the train speeded up on its own or because of something the train's engineer might have done, he said.

Bostian, 32, was among the injured and hasn't yet spoken with NTSB investigators — but he's expected to do so in the next few days, Sumwalt said.

"We want to know his account of what he recalls leading into that tragic event, and then we can start asking specific questions," Sumwalt said.

That could be difficult — Bostian's lawyer told ABC News on Wednesday night that his client has "no recollection of the incident."

What is certain, Sumwalt said, is that the crash wouldn't have happened if the curve had been along a stretch of track equipped with a working Positive Train Control system, which is supposed to be implemented on nearly all U.S. tracks that carry passengers or hazardous cargo by the end of the year.

While much of Amtrak's Northeast Corridor has been been upgraded and turned on, the stretch where Train 188 derailed isn't yet one of them.

Sumwalt and other NTSB officials have taken every opportunity since the derailment to call on lawmakers not to delay full implementation of the system — as has been requested by railroad lobbyists — at a time when rail funding is under attack in Congress.

Sumwalt said it was simple: "One of the four things (Positive Train Control) is designed to do is to prevent overspeed derailments. So here we are looking at an overspeed situation that did derail (the train). I can confidently say that an operational Positive Train Control would have prevented this derailment."